1. Field of the Invention
The present invention relates to a fluorescent X-ray analysis apparatus having means for decreasing a background component of an obtained spectrum by using a polarized light of an X-ray mounted thereon.
2. Description of the Related Art
In a fluorescent X-ray analysis, it is important to detect a fluorescent X-ray generated in a sample, however, even if an X-ray generated in an X-ray tube is detected, the detected X-ray only becomes a background component, which is not necessary for analysis. Therefore, in order to increase an intensity rate (an S/N rate) of a necessary fluorescent X-ray component for an unnecessary X-ray, some methods are employed. Among them, there is a method to use a polarized light of the X-ray. According to a fluorescent X-ray analysis apparatus using a conventional polarized light, an X-ray tube, a secondary target, a sample, and a detector are arranged so as to constitute a polarization optics, which is referred to as a Cartesian arrangement. On the basis of this arrangement, it is possible to prevent that an X-ray generated in the X-ray tube attains to the detector.
Specifically, in a light path of the X-ray passing through the X-ray tube, the secondary target, the sample, and the detector, the following physics phenomenon is generated. This phenomenon will be described along the X-ray light path with reference to FIG. 2. In FIG. 2, directions of x, y, and z; arrangement of components; and a polarization component emitted from the X-ray tube are illustrated, which will be described below.
(1) The X-ray emitted from an X-ray tube 1 is generated when an electron ray is irradiated to a tube target inside of the tube, and the X-ray includes polarization components in two directions, namely, an x direction and a y direction.
(2) In the secondary target, two kinds of phenomena, namely, a phenomenon such that the X-ray emitted from the X-ray tube 1 is scattered and a phenomenon such that a fluorescent X-ray from an element in a secondary target 2, which is excited by the X-ray emitted from the X-ray tube 1, is emitted occur. In the Cartesian arrangement, a sample 3 is disposed in a direction that a scatter angle becomes 90 degrees, so that the polarization component in the x direction is not included in the X-ray which is scattered in the direction of the sample 3. This phenomenon is generated because a refractive index of the secondary target for the X-ray is slightly smaller than the refractive index of air. In addition, the fluorescent X-ray emitted from the element in the secondary target includes the polarization components in two directions, namely, an y direction and a z direction.
(3) In the sample, two kinds of phenomena, namely, a phenomenon such that the scattered X-ray and the fluorescent X-ray emitted from the secondary target 2 are scattered and a phenomenon such that a fluorescent X-ray from the element in the sample 3, which is excited by the scattered X-ray and the fluorescent X-ray emitted from the secondary target 2, is emitted occur. In the Cartesian arrangement, a detector 4 is disposed in a direction that a scatter angle becomes 90 degrees and the direction from the sample 3 to the detector 4 becomes 90 degrees for the direction from the X-ray tube 1 to the secondary target 2, so that the polarization component in the y direction is not included in the X-ray which is scattered in the direction of the detector 4. This phenomenon is generated because a refractive index of the sample 3 for the X-ray is slightly smaller than the refractive index of air. In addition, the fluorescent X-ray emitted from the element in the sample 3 includes the polarization components in two directions, namely, a z direction and an x direction.
(4) The X-ray attaining to the detector 4 only has a polarization component in the z direction of the fluorescent X-ray generated in the secondary target and a polarization component in the z direction and the x direction of the fluorescent X-ray generated in the sample. In the X-ray emitted from the X-ray tube 1, the both components in x and y cannot attain to the detector 4 as a result of scatter twice in the secondary target 2 and the sample 3.
On the basis of the above-described physics phenomenon, the fluorescent X-rays generated in the secondary target and the sample 3 only attain to the detector 4, and a measurement result having very little background can be obtained (for example, refer to U.S. Pat. No. 3,944,822 (see claim 3)).
According to a conventional apparatus, the element in the sample is excited by the scattered X-ray and the fluorescent X-ray emitted from the secondary target. In addition, it is necessary to select a secondary target material so that an energy of the fluorescent X-ray generated in the secondary target is slightly larger than an excitation energy of a sample element so as to increase an excitation efficiency by means of the fluorescent X-ray. On the contrary, in the condition that the energy of the fluorescent X-ray generated in the secondary target and the excitation energy of the sample element are separated or the like, the excitation efficiency of the sample element is remarkably deteriorated. In order to avoid such a condition, preparing many kinds of the secondary targets in response to the sample element, the secondary targets are necessarily used being changed over.
In addition, both of the scattered X-ray and the fluorescent X-ray emitted from the secondary target are diffused in many directions. It is necessary to irradiate most of the X-rays to be diffused to the sample and this involves a problem such that the X-ray irradiation amount is remarkably decreased if a collimator is used in order to measure a minute portion of a sample surface. In the case of focusing the X-rays on the minute portion of the sample surface by using a lens for an X-ray in place of the collimator, there is a problem such that a condition of polarization optics is not met.